The present invention relates to a radio communication system and further relates to a secondary station for use in such a system and to a method of operating such a system. While the present specification describes a system with particular reference to the emerging Universal Mobile Telecommunication System (UMTS), it is to be understood that such techniques are equally applicable to use in other mobile radio systems.
There are two basic types of communication required between a Base Station (BS) and a Mobile Station (MS) in a radio communication system. The first is user traffic, for example speech or packet data. The second is control information, required to set and monitor various parameters of the transmission channel to enable the BS and MS to exchange the required user traffic.
In many communication systems one of the functions of the control information is to enable power control. Power control of signals transmitted to the BS from a MS is required so that the BS receives signals from different MS at approximately the same power level, while minimising the transmission power required by each MS. Power control of signals transmitted by the BS to a MS is required so that the MS receives signals from the BS with a low error rate while minimising transmission power, to reduce interference with other cells and radio systems. In a two-way radio communication system power control may be operated in a closed or open loop manner. In a closed loop system the MS determines the required changes in the power of transmissions from the BS and signals these changes to the BS, and vice versa. In an open loop system, which may be used in a TDD system, the MS measures the received signal from the BS and uses this measurement to determine the required changes in the transmission power.
An example of a combined time and frequency division multiple access system employing power control is the Global System for Mobile communication (GSM), where the transmission power of both BS and MS transmitters is controlled in steps of 2 dB. Similarly, implementation of power control in a system employing spread spectrum Code Division Multiple Access (CDMA) techniques is disclosed in U.S. Pat. No. 5,056,109.
In considering closed loop power control it can be shown that for any given channel conditions there is an optimum power control step size which minimises the required Eb/N0 (energy per bit/noise density). When the channel changes very slowly the optimum step size can be less than 1 dB, since such values are sufficient to track changes in the channel while giving minimal tracking error. As the Doppler frequency increases, larger step sizes give better performance, with optimum values reaching more than 2 dB. However, as the Doppler frequency is further increased there comes a point where the latency (or update rate) of the power control loop becomes too great to track the channel properly and the optimum step size reduces again, perhaps to less than 0.5 dB. This is because the fast channel changes cannot be tracked so all that is needed is the ability to follow shadowing, which is typically a slow process.
Because the optimum power control step size can change dynamically it may improve performance if the BS instructs the MS which value of power control step size it should use in uplink transmissions to the BS. An example of a system which may use such a method is the UMTS Frequency Division Duplex (FDD) standard, where power control is important because of the use of CDMA techniques. Although improved performance can be obtained by having a small minimum step size, for example 0.25 dB, this will significantly increase the cost of a MS. However, if a MS does not have to implement the minimum step size then it may not be able to implement the step size requested by the BS.
An object of the present invention is to enable accurate power control without requiring all mobile stations to implement the same minimum power control step size.
According to a first aspect of the present invention there is provided a radio communication system comprising a primary station and a plurality of secondary stations, the system having a communication channel between the primary station and a secondary station, one of the primary and secondary stations (the transmitting station) having means for transmitting power control commands to the other station (the receiving station) to instruct it to adjust its output transmission power in steps, wherein the receiving station has combining means for processing a plurality of power control commands to determine whether to adjust its output power.
According to a second aspect of the present invention there is provided a primary station for use in a radio communication system having a communication channel between the primary station and a secondary station, the primary station having means for adjusting its output transmission power in steps in response to power control commands transmitted by the secondary station, wherein combining means are provided for processing a plurality of power control commands to determine whether to adjust its output power.
According to a third aspect of the present invention there is provided a secondary station for use in a radio communication system having a communication channel between the secondary station and a primary station, the secondary station having means for adjusting its transmission power in steps in response to power control commands transmitted by the primary station, wherein combining means are provided for processing a plurality of power control commands to determine whether to adjust its output power.
According to a fourth aspect of the present invention there is provided a method of operating a radio communication system comprising a primary station and a plurality of secondary stations, the system having a communication channel between the primary station and a secondary station, the method comprising one of the primary and secondary stations (the transmitting station) transmitting power control commands to the other station (the receiving station) to instruct it to adjust its power in steps, wherein the receiving station processes a plurality of power control commands to determine whether to adjust its output transmission power.
The present invention is based upon the recognition, not present in the prior art, that emulation of small power control step sizes by a MS can provide good performance.